1. Field of the Invention
This invention relates to a coding/decoding method and apparatus therefor for coding a series of data comprising a series of digital bits in order to record digital data on an optical recording medium such as an optical disk or photomagnetic disk.
2. Description of the Prior Art
Various code converting methods used when recording a digital signal on an optical disk or photomagnetic disk or the like have been developed with the progress that has been made in raising recording density and in peripheral techniques such as PLL. Such coding involves converting binary data that is to be recorded into a binary code word pattern suited to the characteristics of the recording/playback system which includes the recording medium.
One type of optical disk on which writing is performed using coded data coded by such a coding method is a photomagnetic disk. With a photomagnetic disk, data is stored by the orientation of magnetization. Specifically, a magnetized film in which magnetization is oriented perpendicular to the disk surface is used as the recording medium. The magnetized film possesses magnetic retentiveness at ordinary temperatures and therefore the direction of magnetization does not change at such temperatures. When the Curie point is exceeded, however, retentiveness diminishes. Accordingly, a strong magnetic field is externally applied to the magnetized film of the photomagnetic disk, in which state the film is irradiated with a laser beam to suddenly raise the temperature of the irradiated portion. When the temperature of the irradiated portion of the magnetized film is thus made to exceed the Curie point, the magnetized film is magnetized in the direction of the magnetic field to perform the writing of data on the photomagnetic disk. When what has been written on the photomagnetic disk is read out, the disk is irradiated with a laser beam weaker than that used for writing and the angle of the deflection plane of the reflected light is detected, thereby determining the magnetization direction of the magnetized film to decide the data.
A sample/hold method has been proposed as a photomagnetic disk tracking servo method. This method is characterized in that reference is made to clock information independent of the written data and preformatted on the disk at a constant interval, whereby a recording code used when recording data does not require a self-clock characteristic. Conventional examples of such a code are referred to as 4/15 (4-out-of-15) and 4/11 (4-out-of-11) codes.
The 4/15 code is one in which original data comprising eight bits is expressed by setting any four bits in a bit pattern of 15 bits to "1". In other words, among the bits set to "1" in a 4/15 code, two are selected from odd-numbered bits and two are selected from even-numbered bits, and in a case where "1"s do not appear consecutively in one code word, is it so arranged that a minimum of two "0"s are inserted between a "1" and a "1". As a result, a bit pattern in which "101" appears in the bit pattern of 15 bits will not occur. Further, it is so arranged that the 15th bit will always be "0".
FIGS. 20A through 20E show conversion tables of such a 4/15 code. When 8-bit data is expressed as a hexadecimal number in these conversion tables, a conversion to a 4/15 code is made in accordance with the tables of FIGS. 20A and 20B if "F" data is not contained as a hexadecimal number in both the higher order four bits [MSN (most significant nibble)] and the lower order four bits [LSN (least significant nibble)]. If the hexadecimal "F" is contained in either the MSN or LSN, then the 8-bit data is converted into 15-bit data in accordance with the conversion tables of FIGS. 20C and 20D.
FIG. 20E shows another conversion rule of the 4/15 code which differs from the rules mentioned above. The 30 code words shown in FIG. 20E are used in special applications, such as for synchronization.
Similarly, the 4/11 code is one in which original data comprising eight bits is expressed by a bit pattern in which any four bits among 11 bits is "1". Part of an example of such a conversion table is shown in FIG. 21.
The characteristics required for such a recording code are as follows:
(1) Minimum magnetization reversal (bit) interval T.sub.min Preferably, T.sub.min is as large as possible to reduce susceptibility to the effects of band limitations in the recording/playback system. If the same degree of waveform interference is allowed, high density recording is possible with the code having the large T.sub.min.
(2) Maximum magnetization reversal (bit) interval T.sub.max In order to obtain a self-clock function, T.sub.max preferably is a small value so as to enable clock information to be extracted from playback data.
(3) Detection window width T.sub.W
This represents degree of margin with respect to time-axis fluctuation such as peak shift due to playback signal jitter or waveform interference. It is preferred that T.sub.W have a large value.
There is also an evaluation approach from (1) and (3) in which a large value for T.sub.min .times.T.sub.W is deemed desirable.
Further, T.sub.ma X/T.sub.min is a parameter which indicates the spread of the signal waveform band. It is preferred that this value be small.
When m-bit data is converted into an n-bit code word in each of the above characteristics, and letting d represent the minimum value and k the maximum value of the run number which is the number of "0"s between "1"s in the code word, a code word in which the value of k is the limit is especially referred to as a run-length limited (RLL) code. In the case of a variable-length code word in which a data word of r.multidot.m-bits is converted into a code word of r.multidot.n bits, the code is referred to as a (d,k; m,n; r.sub.max) code wherein the maximum value of r is r.sub.max. Here m shall be referred to as basic data word length and n as basic code word length. In case of a fixed-length code, the code is indicated with r.sub.max being deleted.
T.sub.max, T.sub.min, T.sub.W are expressed as follows using d and k:
T.sub.min =(d+1).multidot.T.sub.W PA1 T.sub.max =(k+1).multidot.T.sub.W PA1 T.sub.W =(m/n).multidot.T PA1 (T: one-bit length of data word) PA1 T.sub.min =1.07 PA1 T.sub.min =1.45 PA1 (a) (2, 7) RLL (d=2, k=7) PA1 (b) MFM (d=1, k=3) PA1 Y.sub.1 =X.sub.13 +X.sub.14 .multidot.(X.sub.19 +X.sub.20)+X.sub.9 .multidot.X.sub.10 .multidot.X.sub.15 +X.sub.9 .multidot.X.sub.10 .multidot.X.sub.11 .multidot.X.sub.12 .multidot.X.sub.13 .multidot.X.sub.14 +(X.sub.4 +X.sub.5).multidot.X.sub.10 .multidot.X.sub.15 PA1 Y.sub.2 =X.sub.12 +X.sub.13 +X.sub.14 +X.sub.4 .multidot.(X.sub.9 .multidot.X.sub.14 +X.sub.10).multidot.X.sub.15
More specifically, it will be understood that d (the minimum value of the run number of "0"s in the code word) should be enlarged in order to enlarge T.sub.min, k (the maximum value of the run number of "0"s in the code word) should be reduced in order to reduce T.sub.max, and the code word length (n) should be reduced in order to enlarge the detection window width T.sub.W .
In general, in case of a variable code length, a bit error propagates to an ensuing block and word synchronism is lost. Therefore, a word boundary which is easy to locate is an important point in a code word in order to rapidly recover from such a deviation in synchronism.
In the above-described sample/hold method, the clock information is obtained independently of data and the self-clock characteristic is not required of the recording code. Therefore, it can be considered that the limit on higher recording density is decided by waveform interference at the time of playback.
When considering connecting code words following conversion in the case of the conventional 4/15 code or 4/11 code described above, it is found that the minimum value d of the number of "0"s between two "1"s in either code is d =1. The minimum bit interval T.sub.min is
in the 4/15 code and
in the 4/11 code.
However, it has become necessary to raise recording density and to have a large code of the minimum bit interval T.sub.min .
Since the 4/15 code and 4/11 code involve large conversion units, both are disadvantageous in that the ROM capacity used in the conversion circuit and reverse-conversion circuit is large.
Various code conversion methods have heretofore been proposed in view of the foregoing. Typical examples which can be mentioned are MFM, (2, 7) RLL, etc. These are code conversion methods having the features described below. For the sake of convenience, hereinafter codes will be represented upon being normalized at T, and the correspondence between data word and code word is shown in FIG. 4. FIG. 4A shows the correspondence between data word and code word of (2, 7) RLL code, and FIG. 4B shows the correspondence between data word and code word of MFM code. In the latter, X indicates the complement on the preceding bit.
In the (2, 7) RLL code of FIG. 4A, m=1 and n=2. Therefore,
T.sub.min =1.5 PA2 T.sub.max =4.0 PA2 T.sub.W =0.5 PA2 T.sub.min .times.T.sub.W =0.75 PA2 T.sub.max /T.sub.min =2.67 PA2 T.sub.min =1.0 PA2 T.sub.max =2.0 PA2 T.sub.W =0.5 PA2 T.sub.min .times.T.sub.W =0.5 PA2 T.sub.max /T.sub.min =2.0
In the MFM code of FIG. 4B, m=1 and n=2. Therefore,
However, in order to obtain a higher recording density and higher data transmission speed, a larger minimum magnetization reversal interval T.sub.min or a larger T.sub.min .times.T.sub.W and a smaller error propagation, a code conversion method having a word boundary which is easy to find is required.
In addition, a code conversion method is necessary in which the detection window T.sub.W is large to detect data with a low error rate and T.sub.max is small to obtain a stable clock.